Effects of bulk and interfacial charge accumulation on fill factor in organic solar cells

Abstract

The effects of charge accumulation on fill factor (FF) are studied by using light intensity-dependent current-voltage and capacitance-voltage measurements based on poly-{bi(dodecyl)thiophene-thieno[3,4-c]pyrrole-4,6-dione}:1(3-methyloxycarbony1)propyl-1-phenyl [6,6] (PBTTPD:PCBM) solar cells. We find that the FF of un-annealed device shows a non-monotonic dependence of light intensity: first an increase and then a decrease with increasing light intensity. This suggests that both bulk charge accumulation and recombination determine the FF before thermal annealing. Furthermore, the morphological enhancement of PBTTPD:PCBM film leads to a monotonic decrease of FF as increasing light intensity. This indicates that only surface charge accumulation plays an important role in the determination of the FF after thermal annealing.